Plasma is often the preferred specimen type when the goal of laboratory is to reduce the turnaround time and clotting occurences of the analytical systems. Recently, a blood collection tube containing Li heparin as anticoagulant and inert gel material to separate the blood cells from plasma, was introduced for routine laboratory use (Becton Dickinson Vacutainer? Plus, # 7961).We evaluated this plasma separator tube (PST) with Hitachi 917 for direct sampling and storage effect on 27 commonly measured analytes. Blood was collected into PST and serum separator tubes (SST) (Becton Dickinson Vacutainer Plus #7983) from volunteers (n=30), and for additional studies, into Li heparin anticoagulated tubes (PT) (Becton Dickinson Vacutainer Plus #7886). All tubes were centrifuged at the routine setting for the SST processing (RCF=2190xg, 5 min) within 30 min of collection. Samples were analyzed in batch mode from the primary tubes with Hitachi 917 fresh, and after storage for 8 hours at room temperature (rt), and up to 5 days at 4 oC. Significance of the mean difference (mdiff) was based on the imprecision of the method. In comparison with fresh serum, fresh plasma collected in PST had significantly different (mdiff1=plasma-serum) concentration of K (mdiff1=-0.23 mmol/L), albumin (mdiff1=0.1 g/dL), total protein (mdiff1=0.13 g/dL) and activity of ALT (mdiff1=-7 U/L) and LD (mdiff1=25 U/L). None of the measured analytes were affected by the storage of samples in PST and SST for 8 hours at rt. The effect of extended storage at 4 oC (mdiff2=stored-fresh) on plasma analytes of PST specimens was ignificant for bicarbonate (-5 mmol/L), K (0.9 mmol/L), Pi (-0.5 mg/dL), glucose (-7 mg/dL) and LD (39 U/L). In comparison, storage of serum in SST affected only LD activity (-10 U/L). The LD activity for fresh PST plasma was comparable to that for fresh PT plasma (mean: 180 and 174 U/Lrespectively). The extended storage of plasma in PST and PT at 4 oC caused an increase in the activity of LD (PST: mdiff2=30 U/L , PT: mdiff2=24 U/L), indicating incomplete separation of platelets by the gel barrier in PST. However, the PST plasma LD isoenzymes were not affected by storage at 4 oC. In addition, the increase in the PST plasma LD activity was not related to the centrifugation time (5, 8, and 10 min), or to the plasma platelet count. However, we observed that all centrifuged PSTs had varying amounts of erythrocytes imbedded in the gel and/or on top of the gel. This strongly suggests that the observed changes in LD and K were due to incomplete separation of plasma from red blood cells. In conclusion, we found PST to be acceptable for blood collection of heparinized plasma when appropriate changes in reference intervals for ALT, LD, and K are implemented. Furthermore, most of the common analytes are stable for up to 8 hours at ambient room temperature. However, the gel barrier does not provide satisfactory separation of plasma and cells and, therefore, the extended storage of plasma in primary tubes at 4 oC is not advisable.